A. Field of the Invention
The present invention relates generally to the field of wireless communications. More particularly, the present invention relates to improving phase noise performance and receiver/transmitter linearity for a wireless communication system, by using a digital amplitude control loop or a digital voltage control loop.
B. Background
A frequency synthesizer is one of the key blocks in an RF transceiver chip, whereby the frequency synthesizer serves as a local oscillation signal generator. The frequency synthesizer is used to generate a high frequency RF signal by way of an up-conversion mixer in an RF transmitter portion of the RF transceiver chip, and by way of a down-conversion mixer in an RF receiver portion of the RF transceiver chip, thereby performing signal modulation and demodulation. As shown in FIG. 1A, a baseband signal is mixed with a local oscillation signal (LO) in a mixer 110 of an RF transmitter portion 100, whereby an RF signal is output from the mixer 110. This corresponds to up-conversion of a baseband signal to an RF signal. As shown in FIG. 1B, a received RF signal is mixed with an LO signal in a mixer 210 of an RF receiver portion 200, whereby a baseband signal is output from the mixer 210. This corresponds to down-conversion of an RF signal to a baseband signal.
To satisfy the requirement of high data bandwidth in an RF transceiver, such as 54 Mb/s in the 802.11 a/g standard, the phase noise requirement of LO signals become very tight. For example, to maintain a good Error Vector Magnitude (EVM) specification for 54 Mb/s mode in a wide area local area network (WLAN) system with a 3-dB implementation margin, the LO signal's integrated phase noise should be less than 1 degree.
Since there are typically many users in the same band in a wireless system, the interference between users become more serious in a crowded frequency band, which requires each different user sending less interference out of its channel, whereby this system is more immune to other interferences as well. There is accordingly a great demand for higher linearity performance for each single user. Besides designing a high linear mixer, the LO swing should also be optimized since it affects the linearity of the mixer as well.
Furthermore, with the dramatic increase in wireless communications recently, the chip power consumption budget becomes tighter, especially in a portable application. In an RF transceiver, the frequency synthesizer is one of the most power hungry circuits, whereby it burns approximately 40% of the total current consumed by the RF transceiver during receiving or transmitting. Thus, a low power frequency synthesizer is desired for an RF transceiver design.